AIM:To assess the role of IgM and IgG immunohistochemistry(IHC) in the evaluation of autoimmune liver conditions-autoimmune hepatitis(AIH),primary biliary cirrhosis(PBC),and primary sclerosing cholangitis(PSC).METHODS:Forty one biopsies from untreated patients diagnosed with autoimmune liver disease(AIH,n = 20;PBC,n = 13;PSC,n = 8) and fourteen biopsies of patients with chronic hepatitis C were selected.IgM and IgG-positive plasma cells were counted in each sample.RESULTS:A predominance of IgG-positive plasma cells was seen in AIH(90% of cases),PSC(75% of cases),and chronic hepatitis C(100% of cases),while IgMpositive plasma cells predominated in PBC(92.8% ofcases).The IgM /IgG ratio(< 1 or ≥ 1) accurately distinguished PBC from AIH in 90.9% of cases(sensitivity = 92.3%,specificity = 90%),and PBC from either AIH or PSC in 87.8% of cases(sensitivity = 92.3%,specificity = 85.7%).CONCLUSION:Plasmacytic infiltrates expressing predominantly IgM are characteristic of PBC,while other forms of liver disease analyzed in this study,including AIH,typically show an IgG-predominant plasma cell infiltrate.Our data indicate that IgM and IgG IHC may be a useful tool when PBC is a diagnostic consideration. AIM: To assess the role of IgM and IgG immunohistochemistry (IHC) in the evaluation of autoimmune liver conditions-autoimmune hepatitis (AIH), primary biliary cirrhosis (PBC), and primary sclerosing cholangitis (PSC). METHODS: Forty one biopsies from untreated Patients diagnosed with autoimmune liver disease (AIH, n = 20; PBC, n = 13; PSC, n = 8) and fourteen biopsies of patients with chronic hepatitis C were selected. IgG and IgG- positive plasma cells were counted in each sample. RESULTS: A predominance of IgG-positive plasma cells was seen in AIH (90% of cases), PSC (75% of cases), and chronic hepatitis C (100% of cases), while IgMpositive plasma cells predominated in PBC (92.8% ofcases). The IgM / IgG ratio (<1 or ≥ 1) accurately distinguished PBC from AIH in 90.9% of cases (sensitivity = 92.3%, specificity = 90%), and PBC from either AIH or PSC in 87.8% of cases sensitivity = 92.3%, specificity = 85.7%) CONCLUSION: Plasmacytic infiltrates expressing predominantly IgM are characteristic of PBC, while other forms of liver disease analyzed in this study, including AIH, typically show an IgG-predominant plasma cell infiltrate. Our data indicate that IgM and IgG IHC may be a useful tool when PBC is a diagnostic consideration.